Electrolytic reduction cell and collector bar

ABSTRACT

An electrolytic reduction cell for the production of a metal is disclosed. The cell includes a plurality of collector bars ( 21 ). Each collector bar includes an elongated first section ( 27 ) that contacts the cathode ( 15 ) and at least one end section ( 29 ) that extends through one of the cell side walls ( 5 ) and is electrically connected to the electrical current carrier. The cell is characterised in that, for the purpose of controlling current distribution, the first section of each collector bar includes a core ( 31 ) of relatively high electrical conductivity material and an outer housing ( 33 ) of a more mechanically and chemically resistant material than the core material and the end section of each collector bar is formed from relatively low thermal conductivity material.

[0001] The present invention relates to an electrolytic reduction cellfor the production of a metal, such as aluminium.

[0002] The present invention relates particularly to a collector barconstruction for use in such cells.

[0003] Aluminium metal is generally produced in an electrolyticreduction cell by the Hall-Heroult process in which electrical currentis passed through an electrolytic bath comprising alumina dissolved inmolten cryolite to cause the electrodeposition of molten aluminium as ametal pad on the cell cathode. An electrolytic reduction cell comprisesan outer steel shell that is lined with a layer of insulating material,such as refractory bricks. Blocks of carbonaceous material are placed ontop of the insulating layer on the base of the cell and these blocksform the cathode of the cell. The blocks are hereinafter referred to as“cathode blocks”. The cathode must last for the expected operating lifeof the cell, which is typically 1000 to 2000 days. A number ofconsumable anodes are located a short distance above the metal pad thatforms above the cathode. In an operating cell, an electrolytic bath islocated between the metal pad and the anodes, and the passage ofelectrical current through the electrolytic bath breaks down thedissolved alumina in the electrolytic bath into aluminium and oxygen andthe molten aluminium collects in the metal pad on the cathode. Themolten aluminium is periodically drained from the metal pad, typicallyon a daily basis.

[0004] Electrolytic reduction cells are arranged in potlines in which alarge number of cells are connected in series. Electrical current entersa cell through the anodes, passes through the electrolytic bath and padof molten metal and into the cathode. The current in the cathode iscollected and passes to an external current carrier, such as an externalbus bar, and then along to the next cell.

[0005] In conventional aluminium reduction cell technology, collectorbars that are embedded in the cathode blocks are used to collectelectrical current from the cathode and conduct it to an external ringbus. In conventional embedded collector bar technology, the bar is madefrom steel and is either cast or glued into a channel formed in theunderside of a cathode block.

[0006] In an operating cell, the cathode current density distributionalong the length of cathode blocks is uneven with the outermost portionsof the blocks drawing current at up to three to four times higherdensity compared to the inner portions of the blocks. Current travelsunevenly through the cathode blocks as it finds the least resistancepath from the cell. Specifically, current tends to travel through thecathode blocks towards the ends of the collector bars rather thandirectly down through the cathode into the collector bars, thusincreasing the average current path length in the cathode. Poorconductivity of steel collector bars and the use of high conductivitycathode material contribute to the uneven current density.

[0007] One consequence of the uneven current density is an unevencurrent distribution on the surface of the cathode blocks. It is highestnear to the outer edge of the anode shadow or ledge toe. The unevencathode current distribution has a dual effect on cell operation: on theone hand it increases the rate of erosion of carbonaceous material byincreasing the chemical activity of sodium (this drives the aluminiumcarbide-forming reaction) in the affected region; and on the other handit increases the rate of transport of dissolved aluminium carbide byinducing circulation of metal and catholyte. This increased circulationcan result either from increased metal pad heave due to interaction inthe metal pad of horizontal currents with vertical magnetic fields orfrom the Marangonni effect (i.e. circulation induced by uneveninterfacial tension between catholyte and aluminium due to unevencathode current density distribution at the interface). The rate oferosion of carbon is therefore directly related to the current densityand the rate of circulation of metal and catholyte.

[0008] As neither the horizontal currents in the metal pad nor thevertical magnetic fields are even, balanced, or static, their couplingcan lead to hydrodynamic instability of the metal-bath interface. Thecirculation of the metal, the deformation of its surface and theinstability of the metal-bath interface are the three most significantlimitations of the current technology aluminium reduction cells whichaffect potlife (cathode and sidewall erosion) and operating efficiency.Moreover, these limitations make it difficult to reduce theanode/cathode spacing. This spacing has a major impact on the powerrequirements of aluminium reduction cells.

[0009] In conventional aluminium reduction cell technology it isdifficult to have a completely uniform cathode current densitydistribution throughout the cell. The best outcome which can be achievedto date is to reduce the variation of current density distribution byconstructing relatively narrow but long cells having relatively deep,high resistivity, anthracitic cathode blocks and large steel collectorbars. The problem of metal heave and metal pad stability (product offield current interaction) is then addressed through the modification ofbus bars to control the vertical magnetic field. Modern magneticallycompensated cells are a good example of this type of engineering withinthe limitations of the system.

[0010] However, relatively narrow, but long reduction cells are adisadvantage as they have a high external surface to production volumeratio and hence have a high heat loss. Nevertheless, in conventionalcell construction methods, the limitations resulting from embeddedcollector bar technology have been accepted as inherent to the nature ofthe aluminium reduction cells cathode and its negative impact has beenminimised by focussing on improving the magnetic field aspect of thecurrent/field interaction. Modern aluminium reduction cells are designedwith magnetic compensation in order to improve the hydrodynamicstability of the cells, and therefore achieve reductions inanode/cathode spacing. However, this requires relatively expensiveexternal bus bars.

[0011] An objective of the present invention is to improve theefficiency of electrolytic reduction cells by improving the spacialcurrent density distribution in the cells cathode and metal pad.

[0012] According to the present invention there is provided anelectrolytic reduction cell for the production of a metal, which cellincludes: an outer shell and an inner lining of insulating materialwhich form a base, side walls and end walls for containing anelectrolytic bath; an anode; a cathode located on the base of the cell;and a plurality of collector bars which electrically connect the cathodeto an electrical current carrier that is external to the cell, whereineach collector bar includes an elongated first section that contacts thecathode and at least one end section that extends through one of theside walls and is electrically connected to the electrical currentcarrier, and wherein the cell is characterised in that, for the purposeof controlling current distribution, the first section of each collectorbar includes a core of relatively high electrical conductivity materialand an outer housing of a more mechanically stable and chemicallyresistant material than the core material and the end section of eachcollector bar is formed from relatively low thermal conductivitymaterial.

[0013] The applicant has made the following findings in computermodelling studies and in operation of several test cells.

[0014] 1. The use of collector bars having a highly electricallyconductive core improves the spatial current density and therefore thestability of an electrolytic reduction cell.

[0015] 2. The use of collector bars having a relatively low thermalconductivity end section avoids excessive heat loss from the cell viathe collector bars.

[0016] 3. Construction of collector bars with the conductive coreenclosed in a more mechanically and chemical resistant material than thecore material achieves collector bar durability at least equivalent toconventional steel collector bars.

[0017] More particularly, the applicant has found that the use ofrelatively high electrical conductivity material, such as copper, as thecores of collector bars does not have the disadvantages that were foundwith prior art proposals, such as U.S. Pat. No. 3,551,319 of Elliot andare likely to arise with the proposal disclosed in the U.S. Pat. No.5,976,333 of Pate.

[0018] In the Elliot proposal, copper cored bars were originally used toimprove voltage losses but were not applied for commercial productionpurposes. The copper extended all the way to the ends of the collectorbars, ie outside the cell, and the high thermal conductivity copperextracted much more heat than conventional steel bars and resulted in anoverall increased cell heat loss, excessive cell instability and longterm thermal cycling. The result was a reduced performance and overallhigher voltages. The applicant has realised that a significantproportion of the voltage savings that were thought to be possible withcopper cored collector bars can be achieved without having to form thecollector bars with highly electrically (and thermally) conductive endsections outside the cathode. As a consequence, with the presentinvention the applicant has been able to achieve reduced cell overallheat loss and maintain correct cathode heat balance to permit stableoperation. The net effect has been a greater overall energy savingthrough lower voltage requirements and lower energy consumption due torequired current efficiency and controlled current distribution.

[0019] Preferably the core material is copper or a copper alloy.

[0020] Preferably the outer housing material is a relatively lowelectrical conductivity material compared to the core material.

[0021] Preferably the outer housing material is steel.

[0022] Preferably the end section material is steel.

[0023] Preferably the cathode is in the form of a plurality of blocksthat are positioned side by side on the base of the cell.

[0024] More preferably the cathode blocks extend side by side along thelength of the cell with the ends of the blocks contiguous with the sidewalls of the cell.

[0025] In one embodiment there is one collector bar per cathode block,with the first section extending along the length of the block and theend sections of the bar being formed from relatively low thermalconductivity material and extending through opposite side walls.

[0026] In another, although not the only other, embodiment there are twocollector bars per block, with the first section of one bar extendingsubstantially half way along the length of the block with an end sectionextending through one side wall and the first section of the other barextending substantially half way along the length of the block with anend section extending through the other side wall.

[0027] Preferably the undersurface of the block includes a channel whichreceives the first section of the collector bar.

[0028] Preferably the first section of the collector bar is cast orglued in the channel.

[0029] Preferably the cell includes a means for increasing the effectivesurface area of electrical contact between the cathode and therelatively high electrical conductivity material core of each collectorbar.

[0030] Preferably the cell also includes a means for improving both thelongitudinal and transverse distribution of current in the cathode.

[0031] In one embodiment the electrical contact means includes aplurality of electrical contact plugs mounted in electrical contact tothe cathode and to the collector bars.

[0032] Preferably the collector bar is cylindrical and the diameter ofthe core is 60-80%, more preferably 70%, of the diameter of thecollector bar.

[0033] The present invention is based on thermal, electrical and stressmodelling studies on a proposed aluminium reduction cell design and onthe results of operation of test cells based on the cell design at thesmelter of the applicant situated at Bell Bay, Tasmania, Australia. Thecell design is based on the use of collector bars having a copper corehoused in an outer steel sleeve. The cell design is described in moredetail in section D in relation to the figures.

[0034] A. Thermal Modelling of Cell Design

[0035] Thermal modelling of the cell design with a preferred form ofcopper-cored collector bars in accordance with the present inventionpredicted the following:

[0036] 1. The cell design would not incur any thermal penalty because ofthe use of the low thermal conductivity end design of the bars.

[0037] 2. When operated at standard Bell Bay operating conditions with ametal level of around 150 mm there could be a small voltage benefit.

[0038] 3. At a lower metal level higher voltage savings could beachieved.

[0039] B. Electrical Modelling

[0040] Electric modelling of current distribution in the test andconventional cells established that significant improvements in currentdensity distribution can be achieved through the use of copper-coredcollector bars.

[0041] Table 1 contains a compilation of the expected currentdistribution data obtained through electrical (3-D) modelling whichshows that the cell design (“the Test Cell”) had a significantly moreuniform cathode current density distribution and significantly reducedhorizontal currents compared to two standard cells (“Std” and “GraphiticStd”). TABLE 1 Vertical and Horizontal Current Distribution in CellsVertical Horizontal Current Current Metal Distribution DistributionHeight (amp/cm²) (amp/cm²) Cell Design (mm) Ave. S.D. Ave. S.D. Std 1800.756 0.245 0.320 0.166 Graphitic 180 0.744 0.296 0.804 0.188 Std. TestCell 180 0.796 0.106 0.166 0.071 Std 60 0.757 0.229 1.121 0.550Graphitic 60 0.746 0.295 1.329 0.682 Std Test Cell 60 0.795 0.106 0.5470.212

[0042] C. Stress Modelling

[0043] The expansion coefficient of copper is higher than that of steel,leading to differential expansion of copper.

[0044] Consideration of a situation where the copper-core perfectly fitsthe steel tubing indicated the possibility of high hoop stressesdeveloping on the outer surface of the steel hollow. However, themodelling showed that, even under the worst case assumptions, thestresses which could be generated would not exceed the tensile strengthof mild steel. Hence, the modelling showed that cracking of steel isunlikely to be a problem. Under operating conditions and temperature of900° C. both copper and steel are ductile and would easily deform torelieve these stresses.

[0045] Physical modelling of this worst-case scenario—using a sample inthe form of a 150 mm long copper core tightly-fitted into a steel tubeand heated to 1000° C. and held at temperature for 2 weeks—showed thatcracking is not a problem. Electrical resistance testing of theinterface between the copper and steel of the sample indicate a lowcontact resistance of about 0.05 Ωmm² (<1 mV).

[0046] The sample was cut open and the interface between the copper andthe steel was examined using SEM and Microprobe analysis. Theexamination showed the following:

[0047] 1. The interface between copper and steel was subject to oxidepenetration to a distance of 10-20 mm from the ends;

[0048] 2. The oxide combined with the alloying elements in steel (Si andMn) to cause precipitation of oxide particles and grain boundaryembrittlement in steel;

[0049] 3. There was mutual migration of copper and iron across thecopper/steel interface and a metallurgical bond formed in the regions ofinterface which were not affected by oxide penetration;

[0050] 4. Regions affected by oxygen penetration did not form thismetallurgical bond.

[0051] The work established the need for care to exclude the possibilityof air access to the copper/steel interface to avoid deterioration ofcontact resistance. Also, the work also showed that, if exclusion of airis successful, there is a likelihood that a metallurgical bond may formbetween the copper and steel to make this interface more resistant toany attack by sodium in subsequent service.

[0052] D. Cell Design

[0053] The test cells were constructed with collector bars of half-celllength. It is noted that the present invention is not restricted to sucharrangements and extends to full cell collector bars.

[0054] FIGS. 1 to 6 illustrate the construction of one test cell.

[0055]FIG. 1 is a vertical cross-section along the length of the cell,FIG. 2 is an enlargement of the right hand end of the cell shown in FIG.1, FIG. 3 is a vertical cross-section across one half of the cell, FIGS.4 and 5 are longitudinal cross-sections of the collector bar used in thecell, and FIG. 6 is a perspective view of the collector bar.

[0056] The cell has parallel side walls 5 (FIG. 3), parallel end walls 7(FIGS. 1 and 2), and a base 9 (FIGS. 1 to 3). As with conventionalaluminium reduction cells, the test cell is relatively long and narrow.

[0057] The side walls 5, end walls 7 and base 9 include an outer steelshell 11 and an inner lining 13 of suitable refractory material.

[0058] The cell also includes a plurality of cathode blocks 15 locatedon the refractory lining 13 of the base 9 and arranged to extend acrossthe cell to the side walls 5 and side-by-side along the length of thecell.

[0059] The cell also includes a plurality of anodes (not shown).

[0060] Each cathode block 15 is formed with a channel 19 in theundersurface of the block 15. The channels 19 extend along the wholelength of the blocks.

[0061] The cell further includes collector bars 21 which electricallyconnect each cathode block 15 to an external ring bus (not shown). Eachcollector bar 21 includes an elongated section 27 that is cast or gluedin one of the channels 19 in a cathode block 15 and an end section 29that extends through one of the side walls 5 and is connected to thering bus.

[0062] The elongated section 27 is generally cylindrical and has acentral core 31 of copper and an outer sleeve 33 of steel. The terminalend of the elongated section 27 is closed by a steel disc 35. The endsection 29 is generally blocked-shaped and is formed from steel.

[0063] A preferred method of constructing the collector bar 21 (ofpreferred dimensions) is described below.

[0064] 1. Construction of end section 29

[0065] (i) Cut a 370 mm long 100×100 mm steel bar, drill and prepareends;

[0066] (ii) Centrally drill a 70 mm die hole (37 in FIGS. 4 and 5) to adepth of 55 mm;

[0067] (iii) Cut a 45° external bevel (39 in FIGS. 4 and 5) to create agroove for welding;

[0068] 2. Construction of elongate section 27

[0069] (i) Cut a 70 mm diameter×1150 mm long copper rod 31.

[0070] (ii) Slide fit the copper rod 31 into a 1045 mm long steel tubing33 with 100 mm OD and 70 mm ID. Bevel the edges at 45° at a depth of 10mm on one end.

[0071] 3. Assembly of collector bar

[0072] (i) Insert the copper rod 31 into the hole in the 370 mm steelcollector bar and weld copper to steel.

[0073] (ii) Place the assembly into a 200 tonne press and push thecopper into the hollow steel tube until the pressure increases reachesthe press maximum. The copper core 31 should end up being 30-70 mmshorter than the outer steel tube 33.

[0074] (iii) To this end of the assembly weld a steel disc 35, 10 mmthick and 100 mm diameter. Appendix 9 contains the drawings whichdescribe the CCCB assemblies.

[0075] The first test cell was operated for 876 days until it wasdeliberately cut out for autopsy. At the completion of the autopsy thecell was reconstructed and restarted successfully and operates as thesecond test cell.

[0076] The autopsy results indicate that the performance of the testcell was favourable when compared with standard operating cells of theapplicant. Specifically, the test cell had a statistically lower voltage(100 mV on average for the majority of the operating period) than thatof the standard operating cell, a similar current efficiency to thestandard operating cell, and the noise was lower or similar to that ofthe standard operating cell.

[0077] Many modifications may be made to the preferred embodimentwithout departing from the spirit and scope of the present invention.

[0078] By way of example, whilst the preferred embodiment of thecollector bar 21 shown in the figures includes a generally cylindricalcopper-cored elongated section 27 located within the cell and agenerally block-shaped steel end section 29 that extends through theside walls 5 and from the cell, the present invention is not limited tothis construction. The collector bar 21 may be of any suitableconfiguration. By way of example, the collector bar may be generallyflat rather than cylindrical and block shaped. Moreover, the flatcollector bar may have a relatively wide section located in the cell anda relatively narrow section extending through and outwardly from theside walls of the cell.

1. An electrolytic reduction cell for the production of a metal, whichcell includes: an outer shell and an inner lining of insulating materialwhich form a base, side walls and end walls for containing anelectrolytic bath; an anode; a cathode located on the base of the cell;and a plurality of collector bars which electrically connect the cathodeto an electrical current carrier that is external to the cell, whereineach collector bar includes an elongated first section that contacts thecathode and at least one end section that extends through one of theside walls and is electrically connected to the electrical currentcarrier, and wherein the cell is characterised in that, for the purposeof controlling current distribution, the first section of each collectorbar includes a core of relatively high electrical conductivity materialand an outer housing of a more mechanically and chemically resistantmaterial than the core material and the end section of each collectorbar is formed from relatively low thermal conductivity material.
 2. Thecell defined in claim 1 wherein the core material is copper.
 3. The celldefined in claim 1 or claim 2 wherein the outer housing material is arelatively low electrical conductivity material compared to the corematerial.
 4. The cell defined in claim 3 wherein the outer housingmaterial is steel.
 5. The cell defined in any one of the precedingclaims wherein the end section material is steel.
 6. The cell defined inany one of the preceding claims wherein the cathode is in the form of aplurality of blocks that are positioned side by side on the base of thecell.
 7. The cell defined in claim 6 wherein the cathode blocks extendside by side along the length of the cell with the ends of the blockscontiguous with the side walls of the cell.
 8. The cell defined in claim7 wherein there is one collector bar per cathode block, with the firstsection extending along the length of the block and the end sections ofthe bar being formed from relatively low thermal conductivity materialand extending through opposite side walls.
 9. The cell defined in claim7 wherein there are two collector bars per block, with the first sectionof one bar extending substantially half way along the length of theblock with an end section extending through one side wall and the firstsection of the other bar extending substantially half way along thelength of the block with an end section extending through the other sidewall.
 10. The cell defined in any one of claims 6 to 9 wherein theundersurface of the block includes a channel which receives the firstsection of the collector bar.
 11. The cell defined in claim 10 whereinthe first section of the collector bar is cast or glued in the channel.12. The cell defined in any one of the preceding claims includes a meansfor increasing the effective surface area of electrical contact betweenthe cathode and the relatively high electrical conductivity materialcore of each collector bar.
 13. The cell defined in any one of thepreceding claims includes a means for improving both the longitudinaland transverse distribution of current in the cathode.
 14. The celldefined in claim 10 wherein the electrical contact means includes aplurality of electrical contact plugs mounted in electrical contact tothe cathode and to the collector bars.